Earlier this month, she said: "We should be working together to get that right deal for Scotland, that right deal for the UK as I say that's my job as prime minister and so for that reason I say to the [Scottish National Party]: now is not the time."

The disagreement between the pair has already led to a public war of words, with Ms Sturgeon suggesting: "If the Tories refuse to [allow a vote], they would effectively be blocking Scotland's right to choose when the Brexit terms clear.

"This would be undemocratic given [the Scottish government's] clear mandate and also proof positive that the Tories fear the verdict of the Scottish people."

Polls to date have painted an uncertain picture about a second referendum's prospects, although one annual survey showed support for Scottish independence is at its highest ever.

Mrs May is scheduled to have a bilateral meeting with Ms Sturgeon today, and she will meet Police Scotland officers to discuss counter-terrorism issues in the wake of the Westminster terror attack.

She will also give a speech to staff at the Department for International Development, where she will argue: "I believe when we work together, there is no limit to what we can do.

"A more united nation means working actively to bring people and communities together by promoting policies which support integration and social cohesion."

The British government has confirmed that Mrs May will trigger Article 50 on May 29th, which will trigger the beginning of formal negotiations on the UK's exit from the EU.

There is then a two-year deadline to reach a deal, although that can be extended if all parties agree.